Update DescriptorConfig to handle listType fields (APPENG-2852,APPENG-2866)

EC2 images use a listType field for tags. Add special handling for
listTypes to RbuilderCallback and parsing descriptor data into
dictionary.
Fix error in EC2 launch config (APPENG-2852)

Add getChoices to rbuild ui class.

Works just like getChoice, except the user can select multiple options,
acting essentially like a command-line checkbox field.

Add handling for enumeratedType fields to RbuilderCallback

Refactors code to handle enumeratedType fields into a subfunction which
calls into the ui's getChoice or getChoices methods depending on if the
field accepts multiple responses or not.

Use getResponse throughout callback for user input

Updates getChoice and getChoices' shared internal function to use
getResponse, which now takes a "required" argument to indicate whether
to accept empty responses when there is no default set.
Merge branch 'appeng-2852-ec2-security-groups' into feldspar
Merge branch 'appeng-2866-handle-listtype-fields' into feldspar